For micro-machined sensors in single-crystal surface technology it is necessary to mechanically anchor, on the one hand, release-etched structures, such as spring elements or parts of a plate-type capacitor at the substrate and to electrically isolate them therefrom at the same time.
From the printed publication U.S. Pat. No. 5,930,595 “Isolation process for surface micro-machined sensors and actuators” a process is known by means of which silicon structures defined by deep trenches are etched and are also released at their bottom surface towards the substrate by means of a “release-etch” step. The subsequent lining of these trenches with a non-conducting insulating material, such as silicon dioxide leads to a firm anchoring by means of a surrounding of the silicon structure with the lined trenches on three sides, leaving one side uncovered.
This process, does, however, have a number of disadvantages. The lined non-conducting material in the trench has a growth joint in the centre line. Normally, additional voids are created in the inner area of the joint. These effects result in mechanical instabilities and reliability problems.
Furthermore, the anchoring and isolation structure requires an own and comparatively time-consuming etch step for releasing the bottom surface of the structure towards the substrate, the so-called release-etch step. This step is required once again in the later manufacture of the proper mechanical structure, i.e. it is done twice in the course of the process.
From U.S. Pat. No. 6,239,473 B1 an isolation structure for micro-machined arrangements and a process for fabricating the same is known where at first a deep trench with a high aspect ratio is etched into a silicon substrate and is thereafter lined once again with a thermal or deposited silicon dioxide. Thereafter, the trench that is lined in this manner is completely undercut together with the functional structure such that the functional structure is provided with a vertical, electrical insulation against the substrate in the release-etched part. Here, too, a great disadvantage is that when the trench is lined with silicon dioxide, a growth joint as well as voids are formed in its centre which contribute in a not insignificant manner to the instability of the arrangement.